1. Field of the Invention
The invention relates to fiber slurry screening apparatus.
2. Prior Art
Processes for separating contaminants from a pulp fiber slurry are known in the art. In a typical system, pulp-bearing materials and refuse are comminuted and mixed with a liquid, typically water, to form a slurry that becomes the influent to the process. The slurry passing through the screen contains a high proportion of fibrous pulp for further processing. However, the separated material removed by the first screen also still contains appreciable amounts of useable pulp fiber (with a higher proportion of debris than in the original influent). It is advantageous to process the screened out material further to recover some of the useable pulp fiber. Inasmuch as the filtrate that passed the screen likewise may contain particles of debris that were small enough to pass the screen in addition to the desirable pulp fiber, both the filtrate and the separated waste may advantageously be passed through secondary and tertiary screening steps to extract additional pulp fiber from separated debris, as well as to concentrate pure pulp fiber with little or no debris therein.
Various methods and apparatus have been used at secondary, tertiary or subsequent stages in the pulp reclamation process to recover useful pulp fiber from separated debris, while confining and further concentrating the debris. For example, it is known to again mix the debris with a fluid and to utilize a rotary or "drum" screening device for separating out solids at a tertiary stage of a pulp fiber reclamation system. More particularly, an internally fed, rotary drum screening device can comprise a cylindrically shaped screen, often formed of wedgewire.
The screen media comprises openings that are large enough to allow fibrous pulp to pass through, but narrow enough to trap debris in the interior of the cylinder. The drum rests and rotates on trunnions or idler wheels, and is caused to rotate about its central axis by drive means, such as an electric motor. A headbox is suspended in the interior of the rotatable drum and is connected to a source of influent slurry. Influent flows along a trough inside the drum, and spills over side weirs of the headbox onto the rotating screen. The slurry falls across the face of the wedgewire screen that forms the porous wall of the rotating drum. As water and entrained pulp pass through the screen, solids are separated from the slurry, and are caught in and become part of a tumbling mass of debris on the interior surface of the drum. An auger arrangement can be disposed along the interior of the drum and employed to move the entrained wastes axially, to be discharged at an end of the rotating drum. The filtrate and the pulp fibers therein are collected. Some form of cowling collects the filtrate that escapes through the wedgewire, and such drums can be covered by an optional full length hood so as to reduce the amount of splashing of water during the processing.
Various apparatus and methods for screening a suspension of fibrous pulp from a slurry containing undesirable debris using numerous embodiments of a rotary screening drum have been disclosed in the art. Examples include U.S. Pat. No. 4,268,381, issued to Anthony W. Hooper on May 19, 1981; U.S. Pat. No. 4,749,475, issued to Anthony W. Hooper on Jun. 7, 1988; U.S. Pat. No. 4,968,417, issued to Bjorn Ahs on Nov. 6, 1990; U.S. Pat. No. 4,997,558, issued to William H. Baker on Mar. 5, 1991; U.S. Pat. No. 5,008,010, issued to Herbert G. J. Langner on Apr. 16, 1991; U.S. Pat. No. 5,019,248, issued to Andrew F. Kaldor on May 28, 1991; U.S. Pat. No. 5,030,348, issued to Moller Bengt on Jul. 9, 1991; U.S. Pat. No. 5,041,223, issued to Johansson et al. on Aug. 20, 1991; U.S. Pat. No. 5,133,860, issued to Liang C. Tai on Jul. 28, 1992; U.S. Pat. No. 5,433,849, issued to David R. Zittel on Jul. 18, 1995; and U.S. Pat. No. 5,518,614, issued to David R. Zittel on May 21, 1996. Unfortunately, these and other prior art screening devices have provided less than satisfactory results due to their relatively high cost and complexity.
Moreover, it is an aspect of the present invention that the arrangement of a screen media in an internally fed screening drum has been discovered to be an important feature that advantageously can be configured in a manner not previously identified. In a typical drum screen that is adapted to capture solids, the wedgewires are attached inside the drum's rigid frame and run the length of the drum, parallel to the horizontal rotation axis of the drum and spaced from one another around the circumference. This arrangement is designed so that slurry falls onto the screen at a low angle of incidence relative to a tangent, and flows "across the grain" of the wedgewires. Inasmuch as the slurry sluices laterally across the elongated wedgewires and the openings between them, in a shearing motion, solids are unlikely to be caught between adjacent wedgewires and to clog the screen. The flat angle of incidence is such that the flow encounters a relatively reduced slot width compared to the actual circumferential gap between adjacent wedgewires (being effectively about half the actual gap).
Although reducing the tendency to clog is an advantageous feature, it is achieved in a structure that makes the drum screen more of a flow obstruction than it might be. Thus, the low angle of incidence and the flow across the wedgewires reduce the extent to which pulp fiber can be passed through the screen entrained in the liquid filtrate. To achieve a desired rate of flow of filtrate, the influent may need to be pressurized and/or the drum may be vibrated in order to improve the extraction of useable fiber from the slurry.